1. Field
The present disclosure relates to a light-emitting device, and more particularly, to a method and an apparatus for controlling color accuracy of the light-emitting semiconductor-based device within a range of operating conditions.
2. Description of Related Technology
A person skilled in the art will appreciate that the concepts disclosed in this application are applicable to packages for semiconductor-based light-emitting devices, namely a light-emitting diode (LED) device.
To better understand the concepts underlying both the related technology and the detailed description of various aspects of this disclosure, a brief introduction into human light and color perception and one of its scientific representations is provided. The representation well suited for the purposes of this disclosure is a Commission International de I'Eclairage (CIE) chromaticity diagram.
In this disclosure the term light has an ordinary meaning, i.e., an electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye, i.e., (about 400-700 nm, or perhaps 380-750 nm). Similarly, the term color has an ordinary meaning, i.e., a property of light as perceived by a human eye. A person skilled in the art will appreciate that the meaning of the two terms may overlap, e.g., the construct “a light source” and “color source” can be both interpreted as a source of electromagnetic radiation with a wavelength visible to human eye. Should a distinction between these terms be important, the distinction is emphasized in the disclosure.
A connection between human color perception and the CIE chromaticity diagram can be explained by use of tristimulus values. The human eye has photoreceptors, called cone cells, for medium- and high-brightness color vision, with sensitivity peaks in short (S, 420-440 nm), middle (M, 530-540 nm), and long (L, 560-580 nm) wavelengths. Thus, in principle, three parameters, i.e., the tristimulus values, describe a color sensation. The tristimulus values of a color are the amounts of three primary colors in a three-component additive color model and, are most often represented in a CIE 1931 color space, in which they are denoted X, Y, and Z. However, the concept of color can be divided into two parts: brightness (luminance) and chromaticity. For example, the color white is a bright color, while the color gray is considered to be a less bright version of that same white. In other words, the chromaticity of white and gray are the same while their brightness differs. The CIE XYZ color space was deliberately designed so that the Y parameter was a measure of the brightness or luminance of a color. The chromaticity of a color was then specified by two derived parameters x and y, two of the three normalized values which are functions of all three tristimulus values X, Y, and Z.
The CIE chromaticity diagram is thus a two-dimensional representation of the three-dimensional CIE color space, which maps human color perception in terms of the two CIE parameters x and y. Because the normalization has been carried out by further imposing the condition:x+y+z=1  (1)all three normalized values can be unambiguously derived from the CIE chromaticity diagram once a luminance is specified. For further technical description of the CIE chromaticity diagram, see, e.g., “Encyclopedia of Physical Science and Technology”, vol. 7, 230-231 (Robert A Meyers ed., 1987).
FIG. 1 depicts a gamut of human eye visible light spectrum, i.e., all of the visible chromaticities on the CIE chromaticity diagram. The horseshoe shape curve 102 represents spectral (also known as monochromatic) colors, all the remaining colors are not spectral: the bottom straight line 104 connecting the ends of the diagram, thus connecting the two ends of the visible spectrum, is the line of purples, corresponding to the color purple, made up of the two spectral colors—blue (420 nm) and red (680 nm); the interior 106 represents unsaturated colors, i.e., a mixture of a spectral color and a grayscale color.
A classical definition of white light is that of an electromagnetic radiation composed of a distribution of frequencies in the visible range of the spectrum, appearing white to the eye. As such, it is a portion of the unsaturated color region, in particular the line 108. Line 108 called Planckian locus, is the path that the color of a black body takes as the black body temperature changes. The different temperatures are depicted as lines 110 called correlated color temperatures (CCT), i.e., the temperature of the Planckian radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions.
A color near to this locus—a nearly-Planckian white—is characterized by a deviation from the coordinates lying on the Planckian locus 108 and is perceived as a slight coloration called a tint. Thus the distinction between “white” color and other unsaturated colors can be described by specifying the maximum tint permitting the designation “white.” Thus, tint describes degree of departure from a black body concept, and is often prescribed by a standard. By means of an example, the automobile industries standard SAE J578 prescribes the maximum tint permitting the designation “white”; the CIE defines white light as a color point located less than 0.05 in chromatic distance to either the Planckian locus for CCTs <5000K and the standard illuminant locus for CCTs >5000K. Thus the CCT concept is useful in characterizing nearly-Planckian white light sources; such sources are judged by their CCT, i.e., the color temperature of the Planckian radiator that best approximates the nearly-Planckian light sources.
LEDs have been used for many years in various light requiring applications, e.g., signaling states for devices, i.e., light on or off, opto-couplers, displays, replacement of bulbs in flashlights, and other applications known in the art. Consequently, LEDs emitting both spectral colors and white light have been developed. Due to LEDs' advantages, i.e., light weight, low energy consumption, good electrical power to light conversion efficiency, an increased interest has been recently focused on use of LEDs even for high light intensity application, e.g., replacement of conventional, i.e., incandescent and fluorescent, light sources, traffic signals, signage, and other high light intensity applications known to a person skilled in the art. It is customary for the technical literature to use the term “high power LED” to imply high light intensity LED; consequently, such terminology is adopted in this disclosure, unless noted otherwise.
In high power LEDs applications, such as architectural lighting, color is the most important electronic design consideration. Architectural lighting further desires that the applicable LED emit light with particular requirements regarding the color, e.g., white light. Based on the foregoing discussion, the white light color accuracy may be described by desired CCT and maximal deviation thereof, and maximal allowable deviation from Planckian locus. Such requirement is equivalent to defining a point in the CIE chromaticity diagram. Referring to FIG. 2, such a point is referenced as 210. The description of like elements between FIG. 1 and FIG. 2 is not repeated, the like elements have reference numerals differing by 100, i.e., reference numeral 102 in FIG. 1 becomes reference numeral 202 in FIG. 2.
There are two primary approaches to producing white light using LEDs. One is to use individual LED dice that emit the three primary colors—red, green, and blue, and then mix the colors to produce white light. For the purposes of this disclosure a die has its common meaning of a light-emitting semiconductor chip comprising a p-n junction. The other approach is to use a phosphor material to convert monochromatic light from a blue or ultra-violet color emitting LED die or dice to a broad-spectrum white light, much in the same way a fluorescent light bulb works. Regardless of the approach selected, the required white light, represented by chromaticity 210, is achieved by mixing colors from different sources, i.e., the individual LED dice or LED die or dice and phosphors.
One of the properties of the CIE chromaticity diagram is a convenient representation of mixtures of two light sources. Under the laws of color mixture that underlie this system, the chromaticities of all mixtures of any two light sources lie on a straight line connecting the chromaticities of the two light sources. Varying the luminosity of each light source, in other words, varying the ratio of the luminosities varies a particular light defined by a position of a point on the line. By extension, all colors that can be formed by mixing three light sources are found inside the triangle formed by the light sources' points on the chromaticity diagram, and so on for multiple light sources.
Thus, applying the above principle to the three sources of color 212, 214, and 216, the point 210 must lie within the triangle whose vertices are defined by the three sources of color 212, 214, and 216. Consequently, by selecting proper characteristics of the color sources, i.e., position in the CIE chromaticity diagram and luminosity, the required color 210 can be generated.
Application of the concept into practical applications allows for determination of changes in characteristics of the mixed color due to changes in operating condition of the light-emitting device. Although any change in operating conditions is contemplated, in comparison to other light sources, light-emitting diodes generate a significant amount of heat. As a consequence, the operating temperature of the light-emitting device changes due to this internal heating, as well as by any change of the surrounding environment temperature. As the temperature of the light-emitting device increases, the characteristics of each LED changes, e.g., for most blue LEDs, the dominant wavelength increases and the luminous flux decreases. Such a change in the characteristics of each LED results in a change of characteristics of the emitted light as represented, e.g., on the CIE chromaticity diagram. Thus, the change in dominant wavelength is represented by shift in the position of the color source in the CIE chromaticity diagram, e.g., former colors 212, 214, and 216 shift to colors 212s, 214s, and 216s; the change in luminous flux is represented by shift in the mixed color 210s generated by the light-emitting device.
A person skilled in the art will appreciate that although the description above implies LED sources, the mechanism is source agnostic because different phosphor characteristics, e.g., quantum efficiency, emission spectrum, and absorptivity, are functions of the operating conditions, among them temperature. Consequently, change in quantum efficiencies of the different phosphors and the blue or ultra violet color emitting LED die or dice relative to one another results in changes in the emitted light characteristics. Furthermore, to achieve design goals in high power light-emitting devices a combination of LED light sources of different colors combined with phosphors light sources is often used.
By means of an example of such a high power light-emitting device, consider an array of LED dice emitting blue or ultra-violet light, e.g., InGaN dice, encapsulated within a mixture of transparent encapsulant, typically silicone-based, and one or more phosphors, which emit green, red, and/or yellow light upon absorption of the blue or ultra-violet light. However, a conversion efficiency of the red phosphors is lower relatively to the green phosphors. Furthermore, Stokes shift losses are inherent to red phosphors. The luminous efficacy of the red color produced by converting the LED emitted blue or ultra-violet color by the red phosphors is given by the luminous efficacy of the LED emitted blue or ultra-violet color multiplied by the Stokes shift of the red phosphors. Consequently, the Stokes shift losses may negatively affect the luminous efficacy of the red color produced by converting the LED emitted blue or ultra-violet color by the red phosphors. Additionally, red phosphors contain most of the spectral power at shorter wavelengths, i.e., in the near-infrared part of the spectrum. Based on design criteria for the color accuracy, these effects result in limitation on the luminous efficacy of the white light emitted by the light-emitting device.
One configuration solving the above-described limitation on the luminous efficacy is to add one or more red color emitting LED die, e.g., AlInGaP die, to the array of LED dice emitting blue or ultra-violet light. The red color emitting LED die or dice are spectrally narrow compared to red phosphors. Consequently, the red color emitting LED die or dice are spectrally more luminous than the red phosphors; specifically at long wavelengths. Additionally, red color emitting LED die or dice do not exhibit Stokes shift losses inherent to red phosphors. Consequently, for certain design criteria, the added red color emitting LED die or dice compensates for the lower luminous efficacy of the red light produced by converting the LED emitted blue or ultra-violet color by the red phosphors. In an alternative configuration, the red phosphor may be omitted because the efficiency of the red color emitting LED die or dice is sufficient by itself.
In accordance with the above-described principles, such a light-emitting device will exhibit changes in the color accuracy of the emitted light as a function of operating conditions, e.g., temperature.
Although the state of related technology was described in terms of a particular example—a light-emitting device producing a white light, a person skilled in the art will appreciate that such was done for familiarity with the particular example. Consequently, the desired color accuracy is not to be limited by terms CCT and deviation from Planckian locus, associated with white light; after all, as discussed above, these terms describe any area in the CIE chromaticity diagram; therefore, any light.
Accordingly, there is a need in the art for improvements in controlling color accuracy of a light-emitting device comprising at least two light sources within a range of operating conditions, as well as additional advantages evident to a person skilled in the art.